In centrifugal or mixed-flow compressors used for rotary machines, such as an industrial compressor, a turbo refrigerator, and a small gas turbine, improvements in performance are required, and particularly, improvements in the performance of the impeller that is a key component of the compressors are required. Thus, in recent years, in order to improve the performance of an impeller, an impeller in which a recess is provided at a leading edge between tip and hub of the blades to effectively suppress secondary flow or flaking has been proposed (for example, refer to PTL 1).
Additionally, there are impellers (for example, refer to PTLs 2 and 3) in which turbulence is caused in a flow along the hub surface by forming a plurality of grooves in the hub surface between blades such that a boundary layer of the flow along the hub surface is not expanded, in order to improve the performance of a centrifugal or mixed-flow impeller, and in which a plurality of small blades is provided between blades in order to prevent local concentration of a boundary layer.